This invention relates to the co-ordinate addressing of liquid crystal cells. Co-ordinate addressing of such cells can be achieved by methods in which each pixel is defined as the area of overlap between one member of a set of row electrodes on one side of the liquid crystal layer and one member of another set of column electrodes on the other side. In an alternative co-ordinate addressing method the liquid crystal is backed by `an active back-plane` which has a co-ordinate array of electrode pads which are addressed on a co-ordinate basis within the active back-plane, and electrical stimuli are applied to the liquid crystal layer between individual members of this set of electrode pads on one side of the liquid crystal layer and a co-operating front-plane electrode on the other side of the liquid crystal layer. Generally the front-plane electrode is a single electrode, but in some instances it may be subdivided into a number of electrically distinct regions. The active back-plane may be constructed as an integrated single crystal semiconductor structure, for instance of silicon.
This invention relates in particular to the active back-plane addressing of liquid crystal cells which make an analogue optical response to the application of an analogue potential difference across the thickness of the liquid crystal layer. Examples of such analogue liquid crystal effects include the electroclinic effect in the smectic A phase of certain ferroelectric liquid crystal materials. And the distorted helix effect exhibited in certain ferroelectric liquid crystal materials exhibiting a very short helical pitch length typically in the range 0.1 to 0.2 .mu.m.
In the electrical addressing of liquid crystal cells it is generally important to ensure that no pixels are subject to any significant long term cumulative charge imbalance that could give rise to electrolytic degradation effects within the cell In the cases of cells whose response is not polarity sensitive, long-term charge balance can often be ensured by using charge-balanced a.c. stimuli throughout, but clearly there are problems in transferring this approach to the addressing of cells whose response is polarisation sensitive because in these circumstances the application of a charge-balanced a.c. stimulus to a pixel may make it make a temporary excursion from its initial state to some other state, but is then likely to restore it once again to its initial state. The same problem is liable to be encountered in the driving of cells exhibiting an analogue response.
In the ensuing description any particular pixel of a co-ordinate array of pixels is identified by its row and column co-ordinates. Whereas in conventional usage of the terms `row` and `column`, rows and columns are respectively identified as horizontally-extending and vertically-extending lines; in this instance these terms are employed in a wider sense that does not imply any particular orientation of the row and column lines with respect to the horizontal, but merely that the sets of row and column lines intersect each other.